Setup player IDs to be natural array indexes

[audebert]

For a 2 player game, ensure that the first player id is 0, second player id is 1. This will avoid having to implement more get_player_id-type helpers (https://github.com/prologin/prologin2019/blob/091e27c4c172a56ac5d6d3f75f3cf17547fc3de8/src/game_state.cc#L49).

This is a non-trivial change as the server currently allocates player and specator IDs first-come first served and does not know how many player games are for.